Allow MR level approvals_required to be lower than project's

(based on discussion here https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13109#note_180653416)

Currently one can only make approvals required stricter(a higher number) when overriding them in an MR: https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html#overriding-merge-request-approvals-default-settings-premium

However this may be in conflict with the philosophy that overriding is to provide a more permissive mode (see https://gitlab.com/gitlab-org/gitlab-ee/issues/9770#note_144067707)

This constraint can also be sidestepped by providing less approvers. A rule's approvals_required would be capped by the number of approvers available.

By removing the validation related to this, many logic can be simplified, and also allow for an easier API endpoint design.